Large numbers of removable panels are typically disposed about the exterior surface of modern stealthy aircraft, such as access doors and engine bay covers and the like. The panels are typically formed of a metal alloy or composite and conform to the surface contour of the adjacent aircraft skin. A multitude of panel fasteners are required to be used for removably mounting the contoured panels. Such panel fasteners typically comprise a threaded shank and a fastener head. A driving slot or recess is formed in the surface of the fastener head. A mating driving tool engages the recess for fastener installation and removal. The fastener head further includes a conical countersink portion. When installed, the countersink portion of the fastener head is seated within a counterbore formed in the panel, in a position recessed relative to a local external surface plane of the panel. Such configuration allows the surface of the fastener head to be flushed-mounted with the surrounding portion of the panel.
Mitigation or attenuation of the radar cross-section of modern stealthy aircraft is a focus of concern and considerable research and development. Nearly all aspects of the aircraft which contribute to the overall aircraft radar signature is of interest. In this respect, there have been several prior art attempts to mitigate reflected radar emissions or signals associated with the multitude of panel fasteners disposed about the aircraft surface.
It is contemplated that aircraft surface contour irregularities tend to increase radar reflection characteristics. Such discontinuities can occur at the circular interface or surface gap between the panel and the fastener head. As mentioned above, the fasteners may be initially flush-mounted with the surface of the associated panel via a countersink and counterbore arrangement. Such flush-mounting can be disrupted by a common practice of applying a radar absorptive material (RAM) to the panels. Thus, when the fastener is installed, a depression is formed in the surface contour which is above the fastener head the depth of the RAM material applied to the associated panel.
In addition to surface contour irregularities, variations between interfacing materials tend to increase the radar reflectivity. In this respect, when aircraft panels are exposed to electromagnetic energy used for radar detection, localized currents are distributed throughout the panels, which are typically metallic or highly conductive composites. Where these localized currents encounter a significant change in electromagnetic material properties, radar signals may be reflected therefrom. Such a significant change in electromagnetic material properties can arise where there is a void or lack of material, as in the case with the above-described depression above a flush-mounted fastener installed in a RAM coated panel.
Prior attempts to modify the radar cross-section associated with these fasteners include the application of RAM material. Such a process begins with the initial installation of the fasteners. A RAM material is then applied to the surface of the panels and across the fastener heads. The RAM material may be a spray-on type of coating or may take the form of what is referred to as "caulk and tape" type of application. The RAM material is then allowed to cure as required. As a result, a RAM layer is formed which encapsulates the surface discontinuities present at the fastener-to-panel interface and the fastener tool recess. While this process improves the surface continuity characteristics, the application process is undesirably time and labor consumptive and requires specialized training. Further, such RAM materials must be removed to reveal the fastener driver tool recess before the fastener can be removed when it is desired to open the associated panel. Furthermore, upon subsequent closure of the panel, the entire RAM application process must be repeated.
Another prior art approach to modify the radar cross-section associated with aircraft fasteners includes the use of covers or caps which fit atop the fastener head. Examples of this general approach are seen in U.S. Pat. No. 5,603,472 to Hutter, III and U.S. Pat. No. 5,391,028 to Charles. Such covers have a top surface which is configured to align with the adjacent RAM coated panels, and thus improves the surface continuity characteristics thereat. These designs, however, typically call for modification of the driver tool recess formed in the fastener head in order to facilitate the cover attachment. For example, the driver tool recess may be required to include inner threads which engage a plug portion of the cover. As a result, such a radar mitigation approach precludes use of standard fasteners. In addition, the cover must be securely attached to the fastener head in order to prevent undesirable disengagement during aircraft operations. In addition to exposing the fastener head, such disengagement may result in ingestion of the cover by an aircraft engine causing extensive damage. Thus, this approach requires time and labor to attach and inspect the covers. Moreover, such covers must typically be removed in order to remove the associated fasteners. This increases the overall fastener removal time. In some fastener head cover designs, the covers may require specialized driver tools for installation and removal.
Accordingly, there is a need in the art for a fastener treatment which is able to achieve a reduction of radar observability, facilitates ease of fastener installation and removal, and is relatively cost, time and labor efficient.